1. Field of the Invention
This invention relates generally to foam products and, more particularly, this invention relates to chemical blowing agent compositions and methods of making molded foam articles.
2. Description of Related Art
Blowing agents are commonly used either alone or in combination with other substances to produce a cellular structure in a plastic mass. Generally, blowing agents may be classified either as "physical blowing agents" or "chemical blowing agents". Physical blowing agents are generally those materials which are gaseous at temperatures below the processing temperature, i.e., the temperature at which the plastic mass is to be expanded. Chemical blowing agents generally refer to those compositions which decompose or react under the influence of heat to form a gas. Chemical blowing agents range from simple salts such as ammonium or sodium bicarbonate to complex nitrogen-releasing compounds. Chemical blowing agent "concentrates" refer to those compositions and/or components of compositions which are encapsulated in a carrier resin in a high ratio relative to the carrier resin.
Desirable properties of a chemical blowing agent include:
(a) narrow gas-release temperature range, PA0 (b) controllable but rapid gas release rate, PA0 (c) non-corrosive reaction products, PA0 (d) stability in storage, and PA0 (e) reaction products and residue that are compatible with the material to be foamed and have little or no detrimental effect on its properties.
Endothermic chemical blowing agents such as mixtures of sodium bicarbonate and citric acid are gaining in popularity for foaming thermoplastics because of properties such as: fast degassing times, improved cell structure, and endothermic decomposition. However, such sodium bicarbonate and citric acid mixes evolve water upon decomposition which then acts in combination with the unreacted citric acid, often resulting in pitting, rusting and general corrosion of the steel tooling commonly used in injection molding.
Sodium borohydride has found favor as a chemical blowing agent because of its high gas pressure, nondiscoloration of resin to be foamed and non-plating properties. Sodium borohydride, however, requires water or some other proton donor to react with in order to release gas for foaming. Activators based on present technology have not been consistent when used in conventional thermoplastic processes. The effectiveness of known activators, such as aluminum trihydrate and organic acids, (see Hall et al, U.S. Pat. No. 4,413,065) are often dependent upon the type of resin and equipment used. Further, aluminum trihydrate must be heated to a temperature greater than 450.degree. F. to release water to activate sodium borohydride concentrate and many acid activators volatize at a temperature sufficiently near ambient temperature that they are difficult to handle and/or store.
Other hydrated compounds have also been evaluated as chemical blowing agents but these compounds generally suffer from releasing water at so low a temperature that a concentrate of the material cannot be made.